1. Field of the Invention
The present invention relates to an optical pick-up head for recording, reproducing or erasing information with respect to an optical recording medium on which information is recorded with a mark and a space, an optical information apparatus, and an information reproducing method.
2. Description of the Related Art
Recently, as high-density and large-capacity recording media, high-density and large-capacity optical disks called DVDs have been put into practical use, and spread widely as information media that can deal with a large amount of information such as animation.
FIG. 70 shows a configuration of a general optical system used in an optical pick-up head in an optical disk system as an optical information apparatus capable of recording/reproducing information. According to a conventional configuration, an optical recording medium is irradiated with three light beams to detect a tracking error signal (e.g., see pages 5 to 8 and FIG. 2 of JP 3 (1991)-005927 A).
A light source 1 such as a semiconductor laser emits a linearly polarized divergent beam 70 having a wavelength λ1 of 405 nm. The divergent beam 70 emitted from the light source 1 is converted into parallel light by a collimator lens 53 with a focal length f1 of 15 mm. Thereafter, the beam 70 is incident upon a polarized beam splitter 52. The beam 70 incident upon the polarized beam splitter 52 passes through the polarized beam splitter 52 and passes through a quarter-wavelength plate 54 to be converted to a circularly polarized beam. Then, the beam 70 is converted into a convergent beam by an objective lens 56 with a focal length f2 of 2 mm, passes through a transparent substrate 40a of an optical recording medium 40, and is condensed onto an information recording surface 40b. An opening of the objective lens 56 is limited by an aperture 55, and a numerical aperture NA is set to be 0.85. The thickness of the transparent substrate 40a is 0.1 mm. The optical recording medium 40 has an information recording surface 40b. The optical recording medium 40 is provided with a continuous groove to be a track, and a pitch tp is 0.32 μm.
The beam 70 reflected from the information recording surface 40b passes through the objective lens 56 and the quarter-wavelength plate 54 to be converted to linearly polarized light whose plane of polarization is shifted by 90° from an ingoing path. Then, the beam 70 is reflected from the polarized beam splitter 52. The beam 70 reflected from the polarized beam splitter 52 passes through a condensing lens 59 with a focal length f3 of 30 mm to be converted to convergent light. The resultant beam 70 passes through a cylindrical lens 57 to be incident upon a photodetector 30. The beam 70 is provided with astigmatism when passing through the cylindrical lens 57.
The photodetector 30 has four light-receiving portions 30a to 30d. The light-receiving portions 30a to 30d output current signals I30a to I30d in accordance with the respective light amounts received.
A focus error (hereinafter, referred to as a “FE”) signal according to an astigmatism method is obtained by (I30a+I30c)−(I30b+I30d). A tracking error (hereinafter, referred to as a “TE”) signal according to a push-pull method is obtained by (I30a+I30d)−(I30b+I30c). Information (hereinafter, referred to as a “RF”) signal recorded on the optical recording medium 40 is obtained by I30a+I30b+I30c+I30d. The FE signal and the TE signal are supplied to actuators 91 and 92 after being amplified to a desired level and compensated for a phase, whereby focus and tracking control is performed.
When a pitch is reduced so as to increase the capacity of one optical recording medium 40 for recording information, the precision for producing a track also must be enhanced accordingly. However, actually, an absolute amount of error is present, so that when a pitch is narrowed, a production error amount with respect to a pitch is relatively increased. Thus, compared with a DVD, the influence of this error is very large.
FIG. 71 shows a TE signal obtained by scanning the beam 70 in a direction orthogonal to tracks formed on the optical recording medium 40. Tn−4, . . . , Tn+4 on a horizontal axis represent tracks formed on the information recording surface 40b of the optical recording medium 40. In FIG. 71, solid lines extending in a vertical direction represent central positions of the respective tracks Tn−4, . . . , Tn+4 in the case where a pitch tp is formed uniformly. Herein, the track Tn−1 is formed at a position shifted by Δn−1 from a position where the track Tn−1 is supposed to be formed, and the track Tn is formed at a position shifted by Δn from a position where the track Tn is supposed to be formed. Δn−1 is +25 nm, and Δn is −25 nm. As a result, the TE signal exhibits a maximum amplitude a and a minimum amplitude b in the vicinity of the track Tn−1. Thus, the TE signal fluctuates greatly. Furthermore, the position of a zero-intersection point of the TE signal is shifted by an off-track oft1 in the track Tn−1 and by an off-track oft2 in the track Tn from the centers of the tracks Tn−1 and Tn, respectively. More specifically, the off-tracks oft1 and oft2 represent off-track amounts.
Assuming that a fluctuation amount of the TE signal amplitude is defined as ΔPP=(amplitude a−amplitude b)/(amplitude a+amplitude b), and a TE signal is detected by the above-mentioned conventional configuration, the fluctuation amount ΔPP is 0.69, the off-track oft1 is +33 nm, and the off-track oft2 is −33 nm. Thus, the fluctuation amount and the off-track are large. When the fluctuation amount ΔPP of the TE signal amplitude is large, the gain of tracking control is decreased in the tracks Tn−1 and Tn. As a result, tracking control becomes unstable, and information cannot be recorded/reproduced with high reliability.